The Galactic Background Radiation

The ever-present sound of our galaxy

by Dr. Leonard N. Garcia

When making observations of Jupiter you may hear all kinds of radio
frequency interference. Most of the interference will likely be of terrestrial
origin and may be natural, like distant lightning strikes, or man-made
like power line "buzz" or radio stations. There is however a type of interference
that will be inescapable and doesn't come from anything on Earth. It is
the ever-present galactic background radio noise. It can always be heard
but not always at the same strength. The discovery of the origin of this
background noise is usually marked as the birth of radio astronomy.

Karl Jansky, an engineer working for Bell Telephone Labs, was assigned
the task of locating sources of interference in long distance radio-telephone
communications. He built in 1931 in Holmdel, New Jersey an antenna operating
at 20.5 Mega Hertz which rotated horizontally like a merry-go-round. The
rotation of the antenna would allow him to determine the direction from
which this interference was coming. Jansky heard interference from local
lightning storms, interference from distant lightning storms and a third
type of interference, "... a steady hiss type static of unknown origin."
The direction of this third type of interference gradually changed over
the course of a day moving nearly 360 degrees over 24 hours. After months
of careful study of these records Jansky concluded as he reports in a paper
published in 1933, "the direction of arrival of these waves is fixed in
space, i.e., the waves come from some source outside the solar system".
His approximate coordinates for the peak in these radio waves was in the
constellation of Sagittarius, towards the direction of the center of the
Milky Way Galaxy.

On a clear night away from city lights the Milky Way appears to us
as a fuzzy band of light arching across the sky. The Moon, the Sun
and the planets tend to follow a path which intersects this band of light
at two points. When Jupiter happens to cross the Milky Way at one of these
points especially the point that lies closer to the center of the Galaxy
we tend to hear an increase in the galactic background level when we point
our antennas towards Jupiter. The figure below shows a plot of the average
galactic background levels heard during observations of Jupiter at the
University of Florida Radio Observatory. The peaks in the figure in 1972
and 1984 are when Jupiter was in the vicinity of the direction towards
the center of the Galaxy. The spacing of 12 years between peaks corresponds
to the orbital period of Jupiter. The smaller peaks seen in 1977-78 and
1990 correspond to times when Jupiter was crossing the galactic plane far
from the direction of the center. The galactic noise is understood as coming
from high speed electrons spiraling around the weak magnetic fields which
permeate our galaxy.

Left: A plot of the
galactic background antenna temperature (proportional to brightness) at
18, 20, and 22 MHz using Yagi antennas at the University of Florida Radio
Observatory.